Bicycles comprise a frame into which a tubular element (tube sleeve) supporting a saddle and a tubular element supporting a handlebar are inserted. At the point at which the tubular element is inserted, the frame normally presents an axial slot, to the sides of which two projecting lugs are welded, with a screw passing between them. On tightening said screw, the frame diameter is slightly reduced to lock the tubular element at a determined height.
The height (in particular of the saddle) is usually adjusted by slackening the screw engaging the two lugs and acting on the saddle to adjust its height by forcing it to a greater or lesser extent into the frame. Having adjusted the height, the saddle is aligned with the frame by rotating the tubular element which supports it in the frame, until the correct position is achieved. The saddle is then fixed by tightening the previously slackened screw.